This invention relates to a centraliser and relates more particularly but not exclusively to a casing centraliser for facilitating the cementing of casing in a well.
When a well has been drilled for the eventual production of hydrocarbons, one of the procedures commonly employed in readying the well for production comprises placing a hollow tubular casing in the well, and filling the space between the exterior of the casing and the well bore with cement, principally as a sealant and also as a mechanical support. It is desirable that the casing is centralised in the well bore when cemented, and proposals have been made for providing the casing (prior to cementing) with externally mounted centralisers to hold the casing away from the well bore and towards the centre of the bore.
According to a first aspect of the present invention there is provided a centraliser having a body with a bore therethrough for receiving a tubular to be centralised, the body having one or more low friction sliders on the outer surface thereof.
The invention also provides a centraliser having a body with a bore therethrough for receiving a tubular to be centralised, the body having a low friction coating.
The centraliser typically has blades on its outer surface to bear against the wall of a borehole and the slider may comprise the whole or part of a blade. The blades are typically arranged in a peripheral array circumferentially distributed around said body to define a flow path between each circumferentially adjacent pair of blades. Each flow path typically provides a fluid flow path between longitudinally opposite ends of said centraliser, and each blade typically has a radially outer edge providing a well bore-contacting surface. In a simple embodiment of the centraliser the radially outer edge of at least one blade has a low friction coating, strip or block applied thereto by any convenient means.
The inner surface can also have sliders, coatings or strips applied thereto in order to reduce frictional resistance to rotation of the centraliser on the tubular.
The centraliser is preferably a casing centraliser.
The invention also provides a centraliser assembly comprising a centraliser and tubular casing extending longitudinally through the bore of the body. The bore is typically a clearance fit around the tubular casing to be centralised by the centraliser.
The centraliser is preferably free of any means tightly gripping a casing when said centraliser is installed on it, so that the centraliser and casing can rotate relative to one another.
The centraliser body can be made wholly of partially of metals such as Zinc, Steel or Aluminium, or can be of composite materials such as fibreglass, or any other suitable material. We have successfully used the xe2x80x9cZAxe2x80x9d range of zinc alloys supplied by Brock Alloys (GB), and have cast the centraliser body from these materials.
The sliders preferably engage in pre-formed slots or apertures in the body, typically on the outer surface, so that they protrude slightly from the aperture or slot to extend slightly proud of the outermost surface of the centraliser body. The blades are especially good mounts for the sliders, as hollows or slots etc can be readily machined or cast into the material of the or each blade.
The sliders can be of any desired shape but they typically provide a bearing surface with a lower friction coefficient than the body of the centraliser or (in some embodiments) the blades. This enhances the friction coefficient of the centraliser and helps it to slide past obstructions more easily.
The sliders are typically in the form of buttons, patches or strips that are either attached to or inserted into the outer surface of the body, so that they will contact the wellbore or other surface in use before the rest of the body of the centraliser. However the sliders can in certain embodiments comprise the blades with a simple coating of low friction material thereon.
The sliders can be formed from low friction materials to reduce the force needed to slide the centraliser past or along a surface or protrusion, and preferred low friction materials include engineering plastics such as polymeric ethylene compounds, nylon compounds, or any low friction plastics material. Particularly suitable compounds include PTFE, polyetheretherketone, carbon reinforced polyetheretherketone, polyphthalamide, polyvinylidene fluoride, polyphenylylene sulphide, polyetherimide, polyethylene, polysulphone, polyethersulphone, polybutyleneterephthalate, polyetherketoneketone, polyamides, phenolic resins or compounds, thermosetting plastics, thermoplastic elastomers, thermoplastic compounds or thermoplastic polyester resins, PETP, Ketron Peek, Torlon, Nylatron, Ultrawear, and Fluorosint, and their chemical equivalents and related compounds. Preferred coatings include metal/plastic composites such as nickel/phosphorous embedded with PTFE or another low-friction substance.
The blades are preferably equidistantly distributed around the body from one another. They preferably each extend circumferentially at least part-way around the body between longitudinally opposite ends to provide a circumferential distribution of each of the well bore-contacting surfaces. Each blade preferably has a radially inner root integral with the body, and each blade""s root is preferably circumferentially wider than its radially outer edge.
The blades are preferably circumferentially wider at one end (typically the lower end) of the centraliser than at the other (typically lower) end in use. The centraliser preferably has four to six blades.
Longitudinally opposite ends of the blades and/or the body may be chamfered or tapered so as to facilitate passage of the centraliser down a well bore.
Preferably the assembly also includes a centraliser stop collar for longitudinally restraining a casing centraliser when installed on a tubular casing, the stop collar comprising a ring having a substantially cylindrical bore extending longitudinally therethrough, the bore being dimensioned to fit around the casing, and the ring having longitudinal lock means for longitudinally locking the collar to the casing.
The lock means preferably comprises one or more internally threaded bores extending radially through the ring, and a screw-threaded fastener in each internally threaded bore. Each fastener can typically be screwed into contact with the casing to lock the collar in place.
The ring may be formed of any suitable material such as metals like steel, but some embodiments are formed from a zinc alloy which is preferably the same alloy as that from which the centraliser is formed. Each internally threaded bore may be defined by an initially separate thread insert forming an integral part of the collar when fabricated, for example by being cast into the ring, and the thread inserts may be formed of materials which are substantially different from that of the ring, e.g. of brass or steel as compared to a zinc alloy.
Preferably, the centraliser is rotatable on the casing.
The or each centraliser may be longitudinally restrained by a respective stop collar installed upon casing at or adjacent one end of the respective centraliser. One or more centralisers may be longitudinally restrained by a respective pair of stop collars, one of the pair of stop collars being installed on said casing at or adjacent each longitudinally opposite end of the respective centraliser.
The inner surface of the centraliser may have a low friction coating or slider. In some embodiments of the invention the centraliser is coated on its inner and outer surfaces (or on selected parts of these surfaces) with PTFE-impregnated nickel using Niflor(trademark) materials available from Surface Technology plc, preferably using the electroless process known in the art for coating articles with such materials. By slider we mean any member that can present a surface against which the wellbore can bear when the centraliser is in use. The slider can be a button, block or other 3-dimensional object embedded in or adhered to the body or blade, or can be a strip or coating that has negligible or even variable depth. The provision of sliders on the body or blade can be especially beneficial as the sliders can be concentrated on the outermost areas of the body or blade which will have the most contact with the wellbore inner surface, and can therefore be renewed or replaced easily. Indeed, since some areas of the centraliser outer surface can encounter more abrasive conditions than others (e.g. the shoulders of the blades) these can be provided with sliders that are specifically shaped to present the low friction surface of the slider over the whole of the area suffering high abrasion, without having to over-engineer the whole of the body or blade. Also, the sliders on e.g. the shoulders can be made thicker than the sliders provided on less abraded areas of the body or blades e.g. in the middle of the blades, so that the low friction surfaces on the high abrasion areas do not wear out before those on less abraded regions of the centraliser. Therefore, all of the low friction surfaces of the centraliser need not be of the same depth, or shape.